Acqua Alta

High Tides and Flooding in Venice

by Durant Imboden

Acqua
alta, or "high water," can make Venice feel
like Atlantis. At its worst, in 1966, acqua alta flooded the city with more than
a meter of salty lagoon water; more typically, visitors notice water splashing
over canal banks or bubbling up through drains in the Piazza San Marco.

The phenomenon is often mistaken for proof that Venice is sinking. Although the
city did sink about 10 cm in the 20th Century because of industrial groundwater
extraction, the sinking largely stopped when artesian wells on the mainland were
capped in the 1960s.

Today, subsidence is estimated at 0.5 to 1 mm per year,
mostly due to geological factors and compression of the land beneath the city's
millions of wooden pilings.

A larger problem is the rising sea level, which will
become an even bigger threat as global warming melts the arctic ice caps.

Already, the frequency of acqua alta has increased from fewer than 10 times a
year to more than 60 times a year in the last century.

PHOTOS: Intrepid tourists wade in the Piazza San
Marco.

Why (and when) acqua alta floods the city:

Acqua alta occurs when certain events coincide, such as:

A very high tide (usually during a full or new moon).

Low atmospheric pressure.

A scirocco wind blowing up the narrow, shallow Adriatic Sea,
which forces water into the Venetian Lagoon.

By official definition, acqua alta occurs when the tide is 90 cm (35.4
inches) above normal high tide.

The phenomenon is most likely to take place between late
September and April, and especially in the months of November, December, and
October, in that order. The Comune di Venezia's Acqua Alta publication
explains: "In the remaining months, frequency is very low, while there has never
been an event from June to August."

Not all parts of the city are equally susceptible to flooding,
as the following chart from the Comune di Venezia indicates. Also, the actual
depth of water in the streets is far less than the "level of tide" might
suggest. (See the "extreme case" below, where 135 cm of flooding translated into
40 cm of water in the Piazza San Marco.)

Level of tide

Percentage of Venice flooded

Up to 80 cm

Normal tide

100 cm

4%

110 cm

12%

120 cm

35%

130 cm

70%

140 cm

90%

An extreme case:

On the October day when some of the
photos on our main
Venice for Visitors site were taken, the acqua alta reached 135 cm. The Piazza
San Marco was inundated by at least 40 cm or 16 inches of water in what was
billed as the most extreme acqua alta of the decade .

Several times during early winter in the last few years, acqua alta has
reached 150+ cm and flooded most of the city. (The worst deluge in Venice's
history was in 1966, when floodwaters topped out at 194 centimeters.)

Possible solutions:

The Comune di Venezia and various international
organizations have been working on solutions that range from
MOSE Project floodgates at the Lagoon's entrances to raising of pavements in low-lying areas of the city. Much construction
of the latter has already taken place, but it remains to be seen whether acqua
alta can be tamed without closing off the Lagoon from the sea and turning it
into a freshwater lake.